docs: def _check_em_conversion(unit, to_unit=None, unit_system=None, registry=None):
      """Check to see if the units contain E&M units
  
      This function supports unyt's ability to convert data to and from E&M
      electromagnetic units. However, this support is limited and only very
      simple unit expressions can be readily converted. This function tries
      to see if the unit is an atomic base unit that is present in the
      em_conversions dict. If it does not contain E&M units, the function
      returns an empty tuple. If it does contain an atomic E&M unit in
      the em_conversions dict, it returns a tuple containing the unit to convert
      to and scale factor. If it contains a more complicated E&M unit and we are
      trying to convert between CGS & MKS E&M units, it raises an error.
      """
      em_map = ()
      if unit == to_unit or unit.dimensions not in em_conversion_dims:
          return em_map
      if unit.is_atomic:
          prefix, unit_wo_prefix = _split_prefix(str(unit), unit.registry.lut)
      else:
          prefix, unit_wo_prefix = "", str(unit)
      if (unit_wo_prefix, unit.dimensions) in em_conversions:
          em_info = em_conversions[unit_wo_prefix, unit.dimensions]
          em_unit = Unit(prefix + em_info[1], registry=registry)
          if to_unit is None:
              cmks_in_unit = current_mks in unit.dimensions.atoms()
              cmks_in_unit_system = unit_system.units_map[current_mks]
              cmks_in_unit_system = cmks_in_unit_system is not None
              if cmks_in_unit and cmks_in_unit_system:
                  em_map = (unit_system[unit.dimensions], unit, 1.0)
              else:
                  em_map = (None, em_unit, em_info[2])
          elif to_unit.dimensions == em_unit.dimensions:
              em_map = (to_unit, em_unit, em_info[2])
      if em_map:
          return em_map
      if unit_system is None:
          from unyt.unit_systems import unit_system_registry
  
          unit_system = unit_system_registry["mks"]
      for unit_atom in unit.expr.atoms():
          if unit_atom.is_Number:
              continue
          bu = str(unit_atom)
          budims = Unit(bu, registry=registry).dimensions
          try:
              if str(unit_system[budims]) == bu:
                  continue
          except MissingMKSCurrent:
              raise MKSCGSConversionError(unit)
      return em_map